Carrier Sense Multiple Access (CSMA) is a network access method which is frequently used in state of the art computer networks. CSMA methods for conflict resolution are based upon a system in which each conflicting device randomly draws a “back-off” interval within a time period known as a contention window. In a collision detection method, this back-off interval is the time period each conflicting device will wait after the detection of a transmission collision before attempting to access the network medium again. Thus, the device which draws the shortest back-off interval will be the first device to access the network medium after the collision. In a collision avoidance method, the back-off interval is the time period a device will wait after the detection of network activity before attempting to transmit.
Currently, CSMA methods use a uniform distribution of probability to assign a back-off interval to each device directly accessing the network medium. This means that each device accessing the network medium has an equal chance of drawing the lowest back-off interval. However, it can happen that one direct access device accesses the network medium as an intermediary for multiple indirect access devices. In such an instance, even though the intermediary access device is acting on behalf of multiple indirect devices, it still competes with other direct access devices on the network as if it were a single, direct access device. Consequently, devices indirectly connected to the network via these intermediary access devices do not have equitable opportunity to access to the network medium compared to other devices directly accessing the network. This is a problem in wireless or Radio Frequency (RF) networks due the limited available bandwidth and especially in networks using two or more wireless workgroup bridges to link LANs together.
As an example, in a CSMA network with 2 competing direct access devices, each device has a 50% chance of drawing the shortest back-off interval because each device uses the same uniform distribution of probability. The direct access device which draws the shortest back-off interval will be the next to access the network after the back-off interval expires. This is expressed mathematically as:P=1/D where P is the probability of drawing the shortest back-off interval and D is the number of direct access devices attempting to access the network. In the example of 2 direct access devices, the probability is 1/2 that a given device will draw the shortest back-off interval.
If one of the direct access devices is acting as an intermediary network access for N indirect access devices, the number of competing devices is actually 4 (the direct access device and the 3 indirect access devices). Therefore, in this situation, the probability should be:P=(D+N)−1.
Because the intermediary network access device is not attempting to access the network on its own behalf, but is acting on behalf of the N indirect access devices, it should not be a factor in determining the distribution of probability. Therefore, each competing device should actually have a 1/4 probability of drawing the shortest back-off interval. However, due to the uniform distribution of probability being applied to the direct access devices only, each direct access device still gets a 1/2 probability of accessing the network next. Thus, the direct access device has a 1/2 probability accessing the network next while each of the 3 indirect access devices only gets a 1/6 probability of accessing the network next. This discrepancy in network access opportunity increases with the number of direct or indirect access devices in the network.
Therefore, prior art methods for assigning a back-off interval to an intermediary network access device are disadvantageous because they do not give the indirect access devices coupled to it equitable access opportunity to a CSMA network. Specifically, the uniform distribution of probability used to assign back-off intervals does not account for a plurality of multiple indirect access devices which may be competing with direct access devices for network access via an intermediary device. This is problematic because it slows communications for devices which attempt to access a CSMA network via an intermediary device.